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The secretome from mesenchymal stem cells (MSCs) have recently gained attention for new therapeutics. However, clinical application requires in vitro cell manufacturing to attain enough cells. Unfortunately, this process often drives MSCs into a senescent state that drastically changes cellular secretion activities. Antioxidants are used to reverse and prevent the propagation of senescence; however, their activity is short‐lived. Polymer‐stabilized crystallization of antioxidants has been shown to improve bioactivity, but the broad crystal size distribution (CSD) significantly increases the efficacy variation. Efforts are made to crystalize drugs in microdroplets to narrow the CSD, but the fraction of drops containing at least one crystal can be as low as 20%. To this end, this study demonstrates that in‐drop thermal cycling of hyaluronic acid‐modified antioxidant crystals, named microcrystal assembly for senescence control (MASC), can drive the fraction of microdrops containing crystals to >86% while achieving significantly narrower CSDs (13 ± 3 µm) than in bulk (35 ± 11 µm). Therefore, this approach considerably improves the practicality of CSD‐control in drops. In addition to exhibiting uniform release, MASC made with antioxidizing N‐acetylcysteine extends the release time by 40%. MASC further improves the restoration of reactive oxygen species homeostasis in MSCs, thus minimizing cellular senescence and preserving desired secretion activities. It is proposed that MASC is broadly useful to controlling senescence of a wide array of therapeutic cells during biomanufacturing.

This work presents chemically stable and biodegradable hydrogel beads for the isolation of circulating tumor cells (CTCs) and circulating exosomes in liquid biopsy. The liquid biopsy hydrogel beads (^LBbeads) consisting of alginate and poly(vinyl alcohol) hydrogels show both chemical stability and stimuli‐degradable characteristics. Unlike single‐component hydrogels, this hybrid form is not easily degraded by buffers or cell culture media while its degradable characteristic remains; thus, it is useful in bio‐applications requiring multi‐step processes with various reagents and lengthy incubation periods. We applied our platform to clinical samples for isolating two promising circulating biomarkers for a liquid biopsy, CTCs and exosomes, by conjugating the hydrogel surface with anti‐EpCAM and anti‐CD63 antibodies, respectively, thus achieving 37.4 CTCs and comparable amount of exosome recovery per 1 milliliter of blood. The results show easy device‐free isolation and retrieval of CTCs and exosomes, with recovered circulating biomarkers successfully analyzed by western blot analysis and fluorescence microscopy. We believe that this simple and versatile platform enables us to isolate prominent circulating biomarkers for clinical use in cancer diagnosis.